***Whom does it help?
Anyone, who has large data frames or lists which need splitting or column operations.
It is a self-explaining for experienced SQL users. The knowledge is not necessary, but it helps.

***What are the biggest benefits?
It is much quicker than any split / subset and it allows to preform any calculations on a column within one line.
This all is simple, you just have to decide by which column you want to split.
Furthermore, it allows operations "within subsets of a subset" in one line, which safes a lot of coding and thinking (.SD..) This function is hidden in the examples.
Finally, this is not obvious and also a bit hidden in one of the functions: timeline operations are really quick.

***Examples
*subset
If you want a subset of a big list, you converted into a data table just need:
dt[row=rowvalue,] and there are at least two other ways to do it dependent on want you want to do later.
*split
If you want the average of column dependent on values of another one:
dt[,ave(some column), by="other column"] The list will be split or grouped by the individual values of other column.

***Downside
Right now, the documentation favours those, who know SQL very well. However, basic operations are simple and if you just follow the examples and understand that i means row and j means column... For any other step, the learning curve might be a bit flat in the beginning, but it pays off.

***Personal Experience
Once having big loops and splitting, I could reduce the computation time from 62minutes to six...and my code now has only three levels instead of six.